Aggressive Behavior from a year old Chihuahua.

I took in a year old Chihuahua, and now he is being aggressive toward my elderly mother. I can not and will not have that . The pervious owner will not respond to my message to take him back and I don’t want to give him to someone as i’m afraid he might bite someone or a child. The only option I can think of is to take him to the local humane society. Will they put him down? I’m at a loss and not sure what to do. Please give me some advice.

Thanks